1. Field of the Invention
The present invention relates to an apparatus and method for forming functional deposited films and more particularly to an apparatus and method for forming functional deposited films which are suitably usable for preparing an amorphous silicon (hereinafter referred to as xe2x80x9ca-Sixe2x80x9d) photosensitive member for electrophotograph by using plasma CVD.
2. Related Background Art
There have been proposed amorphous materials such as, for example, amorphous silicon compensated with hydrogen or/and halogen as a device member for use in semiconductor devices, light-receiving members for electrophotograph, line sensors for image-input use, image pickup devices, photovoltaic devices, and other semiconductor devices represented by various electronic devices.
Some of these devices, however, have room for improvement in characteristics and production cost of their production. For example, in the production of light-receiving members for electrophotograph, since deposited films of a relatively large area are required to be formed, formation of such deposited films with uniformity in both thickness and electric characteristics and with less defects becomes difficult. Thus there have been demands for a method for forming deposited films for such use which ensures satisfactory productivity and yield.
The deposited films as described above can be formed by the plasma enhanced CVD process, that is, by decomposing a raw material gas with the aid of direct current or high-frequency microwave glow discharge to deposit a thin film on a substrate consisting of a material such as glass, quartz, heat-resistant synthetic resin film, stainless steel, or aluminum. And there have been proposed various apparatus which enable the formation of deposited films which satisfy the above demands.
The apparatus for forming deposited films as described above have, for example, a configuration shown in the schematic view of FIGS. 1A and 1B schematically. FIG. 1A is a schematic transverse sectional view of an apparatus for forming deposited films and FIG. 1B is a schematic longitudinal sectional view of an apparatus for forming deposited films.
The apparatus for forming deposited films shown in FIGS. 1A and 1B includes a reaction vessel 301 having a vacuum air-tight structure. Inside the reaction vessel 301 a plurality of cylindrical substrates 302 for forming deposited films are arranged in such a state that they are mounted on and held by respective substrate holders. Each of the substrate holders for holding the cylindrical substrates 302 can be rotated with a rotating shaft 310 driven by a rotating mechanism 311 consisting of a motor and a gear. Inside the rotating shaft 310 a heater is provided for adjusting the temperature of the cylindrical substrate 302. Further, inside the reaction vessel 301 electrodes 304 for introducing high-frequency electric power and gas introducing pipes 308 for introducing a raw material gas are disposed. To the electrodes 304 a high-frequency power source 307 is connected via a matching unit 306. Each of the gas introducing pipes 308 includes a plurality of openings for emitting gas uniformly and is connected to a raw material gas cylinder via a flow rate control portion (not shown in the figure). One end of an exhaust hole 309 is open to the reaction vessel 301 and the other end communicates with an exhauster (not shown in the figure).
The formation of deposited films by using the conventional apparatus for forming deposited films as described above is performed in the following procedure.
First, the cylindrical substrates 302 each mounted on a substrate holder are installed inside the reaction vessel 301. Then, the reaction vessel 301 is exhausted with a vacuum pump (not shown in the figure) and its internal pressure is decreased to, for example, 10 Pa or less. Then, current is applied to the heater (not shown in the figure) arranged inside each substrate holder to heat each cylindrical substrate 302 to a temperature suitable for depositing films. Once the temperature of the substrates 302 reaches a desired value, a gas for forming a deposited film, such as silane gas, is introduced into the reaction vessel 301 through the gas introducing pipes 308. At the same time, the high-frequency power source 307 is set at a desired electric power to generate high-frequency energy of, for example, 105 MHZ, and high-frequency electric power is introduced into the reaction vessel 301 through the high-frequency matching unit 306, so as to allow glow discharge to occur. This discharge energy serves to decompose the raw material gas introduced into the reaction vessel 301 and form a desired deposited film on each substrate 302. During the formation of deposited films, each rotating shaft 310 is rotated by rotating the rotating mechanism 311 to rotate each substrate 302. Repeating the same operation a plurality of times allows the formation of light-receiving films having desired multilayer structure.
On the other hand, the manufacturing of a-Si photosensitive members (photosensitive members containing a hydrogen-based amorphous material at least in its photoconductive layer) requires advanced technology. Particularly in the photosensitive members for electrophotograph, they require films of larger area and thickness than those of other devices and accordingly, it is an important element how the uniformity of the films is ensured and how the film is formed at a higher deposition rate. From that viewpoint, there have been proposed various methods of manufacturing high quality a-Si photosensitive members in an industrially stable manner. For example, U.S. Pat. No. 6,145,469 (Japanese Patent Application Laid-Open No. 9-310181) discloses a technique of uniformly plasma-treating a substrate of a relatively large area while speeding up the treatment by constituting a part of a reaction vessel with a dielectric member and generating a plasma between a cathode electrode and its counter electrode which are placed interposing the dielectric vessel part therebetween to plasma-treat the substrate in the vessel.
In recent years, however, with increasing demands for electrophotographic apparatus which allow higher-quality images and speed-up, there have also been increasing demands for an apparatus for forming photosensitive members which meets the above demands and has high productivity. Thus the present inventors have concentrated their energy on investigating a method for forming high-quality deposited films at a higher deposition rate using high-frequency electric power with a frequency band of 50 to 450 MHZ. The film formation at this discharge frequency, however, gives rise to a problem such that plasma tends to be maldistributed, and thereby non-uniform treatment is likely to occur in film thickness and quality. This problem is serious particularly when treating a relatively large area, such as of a photosensitive member for electrophotograph. Furthermore, a problem arises such that plasma discharge is difficult to start and the disorder of plasma at the beginning of discharge can sometimes cause deterioration of the whole film and variation in film quality over the whole film.
An object of the present invention is to provide an apparatus and a method for forming a deposited film which allow the formation of a deposited film of a relatively large area at a treatment rate which could not be accomplished by the plasma process of the prior art and allow the stable production of the deposited film without creating variation in the quality.
Another object of the present invention is to provide an apparatus and a method for forming a deposited film, in which the deposited film is formed by providing a dielectric member between at least one high-frequency electrode and a plasma region and applying high-frequency electric power to plasma, whereby a more uniform deposited film can be formed.
Another object of the present invention is to provide an apparatus for forming a deposited film which does not cause contamination of a substrate due to the peeling of the by-products deposited on a high-frequency electrode, thereby allowing the formation of a deposited film of higher quality, which makes it easier to clean the inside of its reaction vessel and to adjust its electrodes, and which is excellent in maintainability.
Another object of the present invention is to provide an apparatus for forming a deposited film which is designed to maintain a desired ratio of the impedance from the surface of its high-frequency electrode to plasma through its dielectric member to the impedance from the surface of its high-frequency electrode to its earth shield, and thereby allows the easy and reliable start of plasma discharge.
Another object of the present invention is to provide an apparatus and a method for forming a deposited film which allow the elimination of the bad effect on an initial deposited film due to the non-smooth start of plasma discharge, and thereby allows the stable production of the deposited film of higher quality.
Another object of the present invention is to provide an apparatus and a method for forming a deposited film which allow the formation of an initial deposited film under more ideal conditions, and thereby allow the improvement in the film quality.
Another object of the present invention is to provide an apparatus and a method for forming a deposited film which allow the stable plasma discharge over a wider range of conditions, and thereby allow dealing with the formation of various types deposited films.
Still another object of the present invention is to provide an apparatus and a method for forming a deposited film which allow the prevention of the maldistribution of plasma probably due to errors in apparatus, for example, an error in setting at each film forming operation, and thereby allow the improvement in uniformity in the quality of a deposited film in the same lot.
A further object of the present invention is to provide an apparatus and a method for forming a deposited film which allow the formation of a deposited film of high and uniform quality on a large area of a substrate at a high deposition rate and allow the stable mass production of a large area of a deposited film excellent particularly in the electrophotographic characteristics.
A still further object of the present invention is to provide an apparatus for forming a deposited film, comprising: a pressure-reducible reaction vessel a part of which is formed of a dielectric member; at least one substrate and raw material gas introducing means both arranged inside the reaction vessel; at least one high-frequency electrode arranged outside the reaction vessel; and an earth shield arranged in such a manner as to cover the reaction vessel and the high-frequency electrode, wherein high-frequency electric power is applied to the high-frequency electrode, high-frequency electric power is introduced into the reaction vessel through the dielectric member constituting the part of the reaction vessel to generate glow discharge in the reaction vessel, thereby decomposing the raw material gas introduced into the reaction vessel, and wherein the following equation (1):
0.8xc3x97(xcex52/d3) less than 1/(d1/xcex51+d2/xcex52)xe2x80x83xe2x80x83(1)
is satisfied where d1 is the thickness of the dielectric member, d2 is the distance from the surface of each of the high-frequency electrodes to the dielectric member, d3 is the distance from the surface of the high-frequency electrode to the inside surface of the earth shield, xcex51 is a dielectric constant of the dielectric member, and xcex52 is a dielectric constant of a space between the reaction vessel and the earth shield. Preferably, the substrate and the high-frequency electrode are provided in plurality, respectively.